The structure of danalite at high temperature obtained from synchrotron radiation and rietveld refinements

The structural behavior of danalite, ideally Fe₈[Be₆Si₆O₂₄]S₂, at atmospheric pressure and from 33 to 1035°C on heating, was determined by using in situ synchrotron X-ray powder-diffraction data (λ = 0.92249 Å) and Rietveld refinement. The sample was heated at a rate of about 9.5°C/min., and X-ray traces were collected at about 18°C intervals. The unit-cell parameter for danalite increases linearly to 1035°C. The percent change in volume between 33 and 1035°C is 1.6(1)%. Other structural parameters show an abrupt change at about 926°C. Between 33 and 926°C, the Be-O and Si-O distances are nearly constant. Between 926 and 1035°C, the Be-O bond length decreases, whereas the Si-O bond distance increases, but the changes are small. From 33 to 926°C, the Be-O-Si angle increases by 1.12(3)°, and the total increase up to 1035°C is 1.21(3 °. Simultaneously, between 33 and 926°C, both the angles of rotation of the BeO₄ 4(ψ_Be) and SiO₄ (ψ_Si) tetrahedra decrease by about 0.71°, respectively. From 926 to 1035°C, both ψ_Be and ψ_Si, increase by about 0.23°. The Fe-S bond length increases by 0.023(1) Å from 33 to 926°C, and by 0.007(1) Å from 926 to 1035°C. The Fe-O bond distance increases by 0.024(1) Å from 33 to 926°C, and decreases by 0.001(1) Å from 926 to 1035°C. Large displacement-parameters occur for the Fe and S atoms, and as the Fe-S bond expands with temperature, the Fe atoms move toward the plane of the six-membered rings in the framework, which causes the tetrahedra to rotate slightly and results in the expansion of the structure. The abrupt change in structural parameters probably occurs because of the oxidation of Fe²⁺ (and minor Mn²⁺) cations beyond 771°C, and the loss of S₂(g) beyond 1029°C.